“Just take a deep breath,” Cowan replied calmly, minutes before being sworn in Thursday by Vice President Joe Biden as the junior US senator from Massachusetts. His interim appointment expires in less than five months, when a special election is held to fill the seat, while most of his colleagues are serving six-year terms. That suits Cowan fine.

Despite entering a body known for egos — with lawmakers putting each vote under the microscope of their political aspirations — Cowan said, “I’m not here to make a mark.”

“I’m grateful for the opportunity to give back to the state that has given so much to me,” he told a gaggle of reporters at the Hart Senate Office Building. “But it’s for the people to decide ultimately who serves them long term in the Senate. I’m not going to take it easy. I’m here to work hard. That’s why the governor sent me here.”

Governor Deval Patrick tapped Cowan to fill the seat last week, choosing his one-time legal counsel and chief of staff over former US representative Barney Frank, among others. Though Cowan has no experience in national politics or lawmaking — as some critics said when the appointment was made — Patrick praised him for his work in Massachusetts during the financial crisis. Patrick added that Cowan “brought cool” to Beacon Hill.

Cowan joins freshman Senator Elizabeth Warren of Massachusetts as Congress navigates the highly-politicized issues of deficit reduction and the debt ceiling. There is plenty of important work to be done in a five-month span, especially for the newest member of Congress, Cowan said.

“It’s like joining a new school, starting at mid-semester,” he said. “But it’s a thrill.”

The 43-year-old stood with Warren at the back of the Senate floor before being sworn in. The pair of rookie lawmakers — only five weeks of Senate experience between them — exchanged a laugh as Kerry joined them.

The crowded Senate gallery fell silent with anticipation as Cowan approached Biden and put his hand on his grandmother’s Bible. After Cowan said, “So help me God,” and shook hands with the vice president, friends and family members above rose to a standing ovation.

Cowan’s wife, Stacy, and their sons, 8-year-old Miles and 4-year-old Grant, beamed as the new senator was congratulated by other lawmakers in attendance. His mother, Cynthia, also joined extended family and friends from Massachusetts to Alabama in the gallery, an aide said. Family members declined to comment.

Patrick came to Washington for the ceremony and congratulated Cowan on the Senate floor. A South Carolina Republican, Tim Scott — also an appointed senator — followed with a handshake and clap on the shoulder. Scott and Cowan represent the first time that two African-Americans have served together in the Senate.

“I couldn’t help but feel a sense of pride for the black community as well as for the state,” said David Thomas, dean of Georgetown University’s McDonough School of Business, who befriended Cowan a decade ago and watched the ceremony from the Senate gallery. “They are both clearly interested in the needs and aspirations of the African-American community, they just have two visions on how to get there. Maybe they can represent the compromise that we need.”

Patrick had said he wanted to appoint a senator who would not run in the June 25 special election; otherwise, he said, a candidate would have to balance campaigning back home with the learning curve on Capitol Hill.

Senate historian Donald Ritchie said the Cowan pick mirrored not only Patrick’s 2009 choice of Paul G. Kirk Jr. to replace the late Senator Edward M. Kennedy, but also the appointment of Benjamin A. Smith II, who was chosen to represent Massachusetts from 1960 to 1962 after his college roommate, John F. Kennedy, became president.

“You want someone who can fit in comfortably both with their [predecessor’s] staff and party,” Ritchie said. “You can be very effective in that period . . . [Cowan] is one of 100 now.”

Among Cowan’s first acts as senator will be to cosponsor the Violence Against Women Act, which was debated on the floor Thursday. He’ll also sign a joint letter with the Massachusetts delegation urging the Commerce Department to aid fishermen in the Northeast.

Cowan and Warren returned Wednesday from a two-day policy retreat in Maryland with Senate Democrats. Along with praising her colleague’s sense of style, Warren said Cowan “hit the ground running.”

“He hit it off with the other senators,” she said. “He is a rare man, indeed, especially in the halls of Washington.”

Not more than a half-hour after being sworn in Thursday, Cowan echoed last week’s news conference, saying, “this may be the shortest political career one can have.” Instead of looking ahead to a political future, he pledged to focus on the policy issues at hand.

Then there is the mundane matter of finding one’s way in the corridors of power. Cowan said he just discovered which elevators senators should use, “so give me time.”